Syncing a Blender camera tracking scene to an Fspy camera match
Looking at your tracked scene it seems to me that you don't have enough parallax to solve the scene using the default settings. There hardly any movement in those trackers, or not enough for blender to determine real 3d information.

Such static scenes should be solved as "tripod". But before, solving the scene use the field of view (or lens and sensor combination) you got from fSpy.

Once the scene is solved as tripod you can manually align it with the existing geometry. Note that tripod solve will not give you accurate 3d but just a visual reference.

Read: How can I get better results when doing camera motion tracking? and Correct tracked camera orientation

Alternatively, given that the camera is not really moving, consider doing a stabilized version of the shot, and use that with the camera from fspy.